1. Field of the Invention
The present invention relates to a prosthesis for the knee articulation.
2. Description of the Related Art
The articulation of the knee allows the leg to move in flexion and extension on the thigh, and is formed between the femur and the tibia, on the one hand by means of their ends, namely respectively the femoral condyles and the tibial plateau, and on the other hand by means of the patella and the femoral trochlea, and ligaments which prevent possible dislocation.
During flexion, the condyles, which are of convex shape, roll and glide over the tibial plateau to allow a degree of flexion which is extensive but nevertheless limited by the soft matter.
Further, flexion is accompanied by axial rotation of the tibia relative to the femur.
In hyperextension, the femur and the tibia are locked in continuation of one another, which makes it possible to maintain posture without fatigue since no muscular effort is necessary.
In the event of an articular injury to the knee, it is sometimes necessary to resort to the fitting of a partial or total prosthetic knee articulation.
The first prostheses for the knee articulation were simple hinges, permitting only a flexion/extension movement by pivoting about a transverse axis, which presented the drawbacks of, on the one hand, an unnatural walking action because the axial pivoting of the tibia relative to the femur is not reproduced, and on the other hand, the fact that complete flexion is not possible.
Most of the knee prostheses known at present include a femoral implant which comprises two condyles and a trochlear shield, the latter being intended to interact with a prosthetic or natural patella, while the two condyles articulate with a tibial implant which includes a plateau covered by a mobile or immobile meniscal element, generally made of polyethylene.
These prostheses are referred to as gliding prostheses, and reproduce the movement of the natural articulation by combining the rolling and gliding of the condyles over the meniscal element.
All of these prostheses have the same drawback, namely a relatively short average life, of the order of ten years, because of the wear to the meniscal element.
The explanation for this is that most existing prostheses include two condyles which are of convex shape and have a radius that is not constant, and a meniscal element which has two concave cavities, also with a radius that is not constant. The contact between the condyles and the meniscal element then takes place linearly, so that during the gliding movement, a phenomenon by which the surfaces of said cavities are abraded takes place over the long term.
In order to lengthen the life of prostheses, some have proposed to distribute the wear phenomenon by creating a second contact surface, through possible gliding of the meniscal element over the tibial plateau.
Others have proposed to separate the twofold movement of gliding and rolling, the gliding taking place between the meniscal element and the tibial plateau and the rolling between said meniscal element and the condyles, the latter having a shape which is at least partially spherical, thus making it possible to obtain partial or total congruence.
However, these prostheses still have drawbacks, in particular in that they are of complex design. Specifically, they involve a large number of parts, which increases the risk of malfunction and requires perfect adjustment, lengthening the operating time to the detriment of the patient.
Prostheses are also known in which the gliding and rolling movements are separated and are respectively fulfilled by the internal condyle and the external condyle. This is the case of the prostheses described in documents U.S. Pat. No. 5,219,362 and German document DE 33 14 038, for each of which the internal condyle is partially spherical and congruent with the meniscal element, while the external condyle can roll over the implant. These prostheses do not allow complete reproduction of the natural movement of the knee articulation, in particular during complete flexion, because the internal and external condyles pivot about the same axis, which furthermore has an effect on the wear to the meniscal element.
If the ligaments are excessively loose, or if the cruciate ligaments are absent, in particular the posterior cruciate ligament, it is necessary to fit a so-called stabilized prosthesis, which is more complex than other prostheses.
Further to the characteristics described above, a stabilized prosthesis generally includes either a centering unit, which may or may not be provided with a spherical head, projecting centrally from the tibial plane and interacting with the femoral implant while passing through the intercondylar notch, or a hook-shaped part against which a transverse bar abuts during flexion, this bar extending through the intercondylar notch and joining the posterior parts of the condyles, or a unit which is arranged in the anterior central region of the tibial plateau and on which the base of the tracheal shield abuts in a hyperextension position.
Further to the complexity of the prosthesis, and the complexity of fitting it, there are still drawbacks of the limitation of the flexion, and above all the risks of detachment of the femoral and/or tibial implant due to repeated shocks when the various moving elements abut against a fixed element.
It will be noted that the prostheses described in the documents mentioned above do not permit perfect stability if some or all of the ligaments are absent, this being especially true of the one in prosthesis described in U.S. Pat. No. 5,219,362 in which the contact surfaces of the condyles are on the same level.